The most obvious reason is to distribute the yeast cells and other ingredients uniformly throughout the dough. One result of uneven dispersal of yeast cells is that the dough will rise faster in some places than in others. It is equally important to develop a firm gluten that will provide a supporting framework for the expanding dough. Gluten is a mixture of proteins in the flour that, when kneaded, become a cohesive network of elastic strands. As the carbon dioxide gas develops, it becomes trapped inside the gluten structure. The trapped gas finds a home in the countless, minute, preexisting air pockets within the dough, or it creates its own minuscule cavities. As more gas develops, pressure builds up within these spaces. This pressure stretches the elastic gluten strands, thus increasing the volume of the dough.
